In recent years, the helical scan CT apparatus comprising the multi-slice function capable of imaging multiple slices of tomographic images at once over a wide range in a short time made possible by a multiseriate function in an X-ray detector has become a main stream in X-ray CT apparatuses. Such X-ray CT apparatuses have facilitated acquisition of continuous data in the body-axis direction of an object to be examined and construction of 3-dimensional images using the acquired data.
These helical scan CT apparatuses have an X-ray tube device including an X-ray tube and its attachments in a scanner rotation unit and an X-ray detector, capable of continuously rotating the scanner rotation unit while continuously moving a table on which the object is placed in the body-axis direction of the object. The helical scan CT apparatus is for relatively effecting helical movement of the X-ray tube device and the X-ray detector with respect to the object by continuous rotation of the scanner rotation unit and continuous movement of the table.
Since the helical scan CT apparatus must continuously irradiate X-rays to the object for a long time from the X-ray tube device installed in the scanner rotation unit, the load on the X-ray tube device increases. When the load increases the heat to be generated from the anode of the X-ray tube also increases, which raises the temperature inside of the X-ray tube.
When the temperature inside of the X-ray tube rises higher than a predetermined temperature, the anode of the X-ray tube needs to be cooled down to a predetermined temperature to prepare for the next imaging. This prolongs the waiting time until the next scanning which lowers the throughput of scanning. The time for cooling the X-ray tube device is more likely to be prolonged, since there is a demand for further improvement on CT image quality which increases the X-ray amount for irradiation.
In this way, improvement of imaging throughput and image quality is highly desired particularly in helical scan X-ray CT apparatuses, which demands large capacity function of the X-ray tube device.
While current of electricity between the anode and cathode of the X-ray tube (hereinafter referred to as tube current) can be increased when the X-ray tube has large capacity function, there is a need to take sufficient measures against discharging in the X-ray tube and the peripheral equipment. Identifying a discharging part is crucial for taking appropriate countermeasure against the problem of discharge.
Given this factor, it is important to identify where in a high-voltage generating device, X-ray tube and high-voltage cable a discharge occurred in order to cope with the problem appropriately. As for the technique for identifying a discharging part, the following technique is disclosed in Patent Document 1. A first resistor for current detection is series-connected to the anode where the X-ray tube is earthed. A second resistor for current detection is series-connected also to the secondary side of the high-voltage generating device. Each output of the first and second resistors for current detection are compared with a predetermined threshold value in a comparison circuit. By such configuration, when a discharge occurs in the high-voltage unit, the portion where the discharge occurred is identified by differentiating the internal X-ray tube from the other part.
Patent Document 1: JP-A-2000-215997
However, in the technique disclosed in the Patent Document 1, when a discharge occurred in the X-ray tube, the space between the anode and cathode of the X-ray tube is short-circuited, and high voltage of direct current in the range of 50 kV˜150 kV which is an output voltage of the high-voltage generating device is directly applied to the first and second resistors for current detection.
For this reason, in order to avoid damage of the first and second resistors for current detection, the resistors need to be formulated with high-voltage insulation to withstand high voltage. Also, the resistors for current detection have to bear a large amount of short-circuit current since resistance value of the resistors for current detection is very small. Therefore, the resistors for current detections turn out to be very large in size, which is a disadvantage for an X-ray CT apparatus where the size and weight of the resistors must be reduced to be mounted in the scanner rotation unit.
Also, there is a possibility that the anode itself of the anode-earthed X-ray tube becomes high potential with respect to the earth potential, which could cause the problem that the detection circuit becomes inoperative and identifying the discharging part becomes difficult. These problems are also common for the cathode-earthed X-ray tube.